1. Related Application
This application is related in subject matter to application Ser. No. 275,044 by the same inventor, bearing the same title; which issued Nov. 22, 1988 as U.S. Pat. No. 4,839,617.
2. Field of the Invention
This invention relates to a bandpass filter circuit arrangement comprising first and second parallel-resonant circuits each of which comprises an inductive component and a capacitive component, each said capacitive component comprising a pair of capacitors in series, corresponding first capacitors of each pair being variable in step with each other, a bidirectional reactive coupling being provided between a point on the connection between the capacitors of one pair and a point on the connection between the capacitors of the other pair.
3. Description of the Prior Art
A filter arrangement of this type is disclosed in patent specification GB-A-1 295 850 (FIG. 4 as modified by FIG. 5 and the corresponding description), to which U.S. Pat. No. 3,624,514 corresponds. In this known filter the bidirectional coupling includes a series capacitor and each variable capacitor is varied in step with variation of tuning capacitors which tune the respective resonant circuits, in order to obtain substantially constant matching of the filter to the output impedance of a transistor which feeds it, over the complete tuning range.
Bandpass R.F. filters are used extensively in radio communication equipment for many purposes such as the rejection of spurious response frequencies in superhet receivers, suppression of unwanted radiation from oscillators, impedance transformation, etc. A typical such filter consists of two or more parallel resonant circuits, with a bidirectional coupling between each resonant circuit and the next. Many types of coupling are possible, and are well documented in the standard reference books. The most commonly used, because of its simplicity and low cost, is so-called "top capacitance coupling" in which one common point of the capacitive component and the inductive component of each resonant circuit is connected to the corresponding point on the next resonant circuit (if present) via a series-capacitor (the other such common points being connected to ground). The value of the (or each) series capacitor is chosen so that the required degree of coupling is obtained; usually this means that the value of each series capacitor has to have a particular relationship to the value of the capacitive component of each of the two resonant circuits which it intercouples, for example to obtain so-called "critical coupling". This means that if a filter which employs simple top capacitance coupling is required to be tunable over a substantial frequency range, for example a frequency range exceeding 10% of a frequency within this range, the tuning has to be done by adjusting the inductive components of the various resonant circuits, e.g. by means of adjustable ferrite cores, because adjustment of the capacitive components would result in an impermissible change of the degree of coupling from the optimum. Thus variable capacitance diodes cannot be used for tuning such filters over a substantial frequency range; if they were, a filter which is optimally coupled in the centre of the tuning frequency range would be substantially overcoupled at the upper end of this range and substantially under-coupled at the lower end.